Orthopedic procedures such as, for example, Vertebroplasty or Kyphoplasty include injection of setting material while they are still in an un-set condition. Setting of the material prior to completion of the procedure can delay completion of the procedure and/or cause medical complications.
Typically, bone cement employed in Vertebroplasty and/or Kyphoplasty comprises an acrylic mixture including a polymer component and a monomer component (e.g. polymethylmetacrylate [PMMA] and monomethylmethacrylate [MMA]). Acrylic bone cements generally set, or harden, rapidly after mixing of the polymer and monomer components. The short amount of time between mixing and full setting defines a “window” of time during which the material must be prepared, loaded into an appropriate delivery device and delivered into the subject. For standard acrylic bone cements, this window is only a few minutes long.
A window of time which is too small can be inconvenient, for example if a long procedure is planned (e.g. treatment of two or more vertebrae in a single operation) and/or if an unplanned delay occurs.
In some medical procedures, high viscosity cements are employed. High viscosity at the time of injection can contribute to a reduction in the risk of cement leakage, while sustaining an ability to infiltrate into the intravertebral cancellous bone (interdigitaion) [see G Baroud et al, Injection biomechanics of bone cements used in vertebroplasty, Bio-Medical Materials and Engineering 00 (2004) 1-18].
In some cases, cements characterized by a high viscosity at the time of injection will set shortly after reaching the high viscosity.
U.S. application Ser. No. 10/549,409 to Ferreyro-Irigoyen et al. describes maintaining a bone cement loaded syringe in a cold atmosphere to slow time of solidification of the cement. The disclosure of this application is fully incorporated herein by reference.
It is known in the art, that reducing the temperature of a polymerization reaction reduces the polymerization rate. In the context of bone cement, this principle has led to the practice of cooling one or more of the polymer component and the monomer component prior to mixing. Cooling is typically done in a refrigerator. Generally, the refrigerator is located outside the operating theater where the cement components are typically mixed. Cooling of components of the polymerization reaction mixture prior to mixing can delay polymerization to a limited extent, however the delay is uncontrollable once mixing begins and the amount of delay cannot be accurately predicted.
If a refrigerator outside the operating theatre is used, warming of the components can occur while they are being moved from the refrigerator to the operating theatre.
Additionally, since polymerization reactions are typically exothermic, any advantage offered by cooling mixture components prior to mixing is typically lost once the polymerization reaction begins to generate heat.